1. Field of the Invention
This invention relates to squelch circuitry used in radio receivers to cut out noise between intervals of signal transmission, and is directed particularly to improvements on such circuits wherein the squelch time delay is such that no squelch noise burst is audible at the end of a received transmission under strong signal conditions.
2. Description of the Prior Art
In radio receivers, particularly in high gain communication receivers, it is common practice to employ squelch circuits operative to automatically block off the audio output in the absence of transmitted signal intelligence or carrier modulation, so that annoying receiver noise will not be heard during intervals between signal reception. Upon the resumption of a signal being received after an interval of silence, the squelch circuit opens the audible path again, allowing the signal to be heard as long as it is present. Prior art squelch circuits, however, are deficient in various respects principally in that they operate with an inherent time delay. In mobile radio communication, particularly when either or both mobile transmitting and receiving units are moving, the level of signal reaching the receiver varies up and down, such variation being commonly referred to as "flutter" or "fades". Such rapid signal strength variation is also frequently caused by atmospheric conditions.
In prior art squelch circuits a long delay time constant is utilized to permit the signal to vary up and down rapidly before the squelch has time to operate thereby avoiding chopping holes in the speech transmission being received in the presence of rapid flutter. While such use of long time delay effectively prevents loss of signal or partial loss of signal under conditions of flutter, there is the advantaage that when signals are at a constant strong level a long noise burst is heard at the end of each transmission interval before the squelch circuitry has had time to operate. Since this noise burst or squelch tail is very annoying at high signal strengths, it is desirable to eliminate this annoying burst of noise during strong signal conditions. One prior art approach is to use circuitry which provides a dual time constant where the time constant is relatively long for weak signals and considerably shortened for strong signals. Another type of squelch circuitry utilizes a variable approach, where a relatively long time constant is provided at weak signals but the time constant varies proportional to the RF signal strength until it reaches a considerably shortened time constant under strong signals. Unfortunately, such prior art squelch circuitry samples noise in the audio range of frequencies and as a result they are not able to provide a sufficiently short time constant under strong signal conditions to completely eliminate the annoying noise burst under such signal conditions.